UPON PLANT-HOUSES. 91 



quality of matter, light itself is neither a universal nor a direct 

 stimulant of vegetable life. 



Since plants do not see that light which causes their growth ; 

 there must necessarily exist, to them, a sort of dark light ; in 

 other words, the light which we see and feel can only exercise an 

 indirect power on the vegetable creation. Plants have no nervous 

 system, and what I call their soul is nothing hut their 

 inherent nisus formatlviis. Hence, they are not susceptible of 

 any real dynamic irritation that is unaccompanied by mechanical 

 or chemical power. Light must consequently operate differently 

 upon them by something dififerent from its luminous quality. A 

 plant which turns towards the light reacts on its dark, not its 

 luminous power. To the question, is light per se cold or warm, 

 I reply it is neither ; it possesses no temperature, but it makes it. 



Ingenhouss has long ago laid down the maxim, that solar light 

 is hurtful to the commencing germination in proportion as it is 

 beneficial to the growing plant. Meese, A. v. Humboldt, and 

 many others have corroborated it, and so does our daily experience. 

 The bud, however, is accelerated in its development by the 

 influence of light, especially direct from the sun, and this appears 

 in a more marked manner in the covered (gemmae perulatse) than 

 the naked buds. As a general law, subject of course to partial 

 modifications, our most eminent Treviranus has stated, in concise 

 terms, that the ascending axis, the upper surface of the leaf, and 

 the flower, require the stimulus of light; whereas the descending 

 axis, the under leaf-surface, and the fruit either require it not, or 

 are even injured by it. In conformity with these postulates we 

 must construct our glass-houses, and I believe a strong reflection 

 of light is better than a weak one, and that a bluish durable 

 colour is preferable to others. 



According to Aubert du Petit Thenars and his followers, it is 

 the leaves that build the tree ; a truth which might be modified by 

 saying that light, supported partially by heat, builds the tree. 

 We must, accordingly, provide as much, and as diffused light as 

 possible in our houses, in order that the contents may spread their 

 branches symmetrically in all directions. It is a matter of course, 

 that a proportionate degree of heat is likewise indispensable. All 

 this is plain enough, when the question is simply the cultivation 

 of trees and shrubs for the sake of exhibiting their natural habitus 

 to greatest advantage, but since we have to provide also for their 

 periods of flowering and fruiting in perfection, certain points are 

 to be considered. Those periods may be fitly divided into four 



